This invention relates to active target classification and more particularly to the apparatus and method for obtaining improved bearing estimation of extended targets.
Range-bearing maps of signal returns are often used for extracting information about the target aspect shape and length. Target imaging for such feature extraction tasks is usually done with high frequency sonars using high spatial resolution beams and short continuous wave pulses. However, the absorption losses at high frequency attenuate the signal rapidly and preclude long range operations. To overcome these problems, detection sonars use low frequency in conjunction with large time bandwidth (WT) signal waveforms such as linear frequency modulated waveforms. The rationale for using the large WT waveforms is when the signal returns are processed through a matched filter, the desired target echo is emphasized, while the effect of noise is minimized. This results in enhancement of the signal-to-noise ratio (SNR) needed for detection and estimation of target parameters. The lack of adequate range resolution causes the sonar echo to be a composite of overlapping returns which tend to interfere with each other. As a result, signal returns from a multi-highlight target in a multipath environment encounter amplitude scintillations. Bearing estimation in such cases is accompanied by large variances called "glint error" in addition to angle error due to additive noise. This invention describes a signal processing technique which overcomes these problems by optimal combination of temporal and spatial processing to extract accurate range-bearing estimates for target imaging.
There exists the need for improvements in active target classification capability of sonar systems which are presently available. Of fundamental importance to the classification task is the extraction of information about the target aspect, shape, and length. Suggested systems for providing this information use high resolution pulse compression and split beam processing to provide information to a two-dimensional display of range versus cross range of the target in a 500 yard to 1000 yard window. The basic limitation of this prior art processing is the bearing estimation of the extended target which provides sonar signals having multiple highlights in a multipath environment. Bearing estimation in observing such targets is accompanied by "glint error" due to interference among the multiple returns, in addition to angle error due to noise.
The conventional prior art approach of high resolution bearing estimation is shown in FIG. 1. FIG. 1 shows a system 10 in which a sonar array 11 of transducers provides, in a manner well known to those skilled in the art, phase separated beams 12, 13 with aperture spacing D which provide target and noise signals S(t)+N.sub.1 (t), S(t+t.sub.0)+N.sub.2 (t), respectively, on lines 14, 15. The signals on lines 14, 15 are applied to their respective matched filters (replica correlators) 16, 17 which provide correlated complex output signals C.sub.R, C.sub.L, respectively. The complex signals C.sub.R, C.sub.L correspond to the signals provided by the right and left phase-center displaced beams 12, 13, respectively. Cross correlation of the complex signal C.sub.R with the complex conjugate (C.sub.L *) of signal C.sub.L in the cross correlator 18 provides the real part of the cross correlated C.sub.R, C.sub.L * signals on line 19 and the imaginary part of the cross correlated C.sub.R, C.sub.L * signals on line 20. The real and imaginary signals are applied as inputs to an arc tangent circuit 21 which operates on the ratio of the imaginary to the real part of the output of cross-correlator 18 to provide the phase angle .o slashed. (the argument) on line 22. The phase angle .o slashed. is converted to a bearing .THETA. in converter 23 by dividing the angle .o slashed. by the Horton phase factor (2.pi.D/.lambda.) where D is the phase center separation of the beams 12, 13 and .lambda. is the average wavelength of the sonic energy which is received in the beams 12, 13. The cross range value is provided on line 24 by multiplication of the bearing .THETA. by the range of the corresponding signal within converter 23 thereby providing an electrical signal to the X axis deflection circuitry to provide the cross range deflection on the range/cross range display unit 25.
A system trigger causes the transmitter 26 to initiate a linear frequency sonic signal for propagation by the transducer array 11 to provide acoustic energy over a region to which the phase displaced beams 12 and 13 are responsive to reflected signals. The system trigger is also provided to initiate a Y axis deflection circuit 27 which provides a range ramp voltage to the Y deflection circuitry of the range versus cross range display 25. The real part and imaginary part of the signals on lines 19 and 20, respectively, are provided to an amplitude generating circuit 28 which takes the square root of the sum of the squares of the real and imaginary parts of the signals on lines 19 and 20 to obtain the magnitude of the received signal. The output of amplitude generating circuit 28 is provided to a detector 29 to which a threshold voltage is also applied. When the magnitude of the signal provided by circuit 28 exceeds the threshold, an output voltage is provided on line 30 which provides an intensification signal to the display 25. Where the display 25 is a cathode ray tube display, the intensification signal would be provided to a grid controlling the electron beam striking the face of the cathode ray tube. For each signal detected by detector 29, a spot would appear on the face of the display 25 with the Y and X location of the spot being determined by the range and cross range provided by circuits 23 and 27. A typical display provided by the prior art circuitry will be presented later as FIG. 6 for comparison with displays provided by the system of this invention.